1. Field of the Invention
The present invention relates to a magnetic bearing apparatus for raising a spindle of a machine tool or the like to the surface to control the position thereof.
2. Description of the Related Art
There has been conventionally a magnetic bearing apparatus so adapted as to support a rotating member, for example, a rotor of a spindle of a machine tool by magnetic forces of two magnetic bearings disposed in two positions in the axial direction with clearances between the magnetic bearings and the rotor, detect the position of the rotor by a position sensor, and control a current supplied to electromagnets in the two magnetic bearings on the basis of a detection signal of the position sensor, thereby to hold the rotor in a reference position (a target position).
Furthermore, in the above described control, the translation motion and the tilt motion concerning the center of gravity of the rotor are separately controlled.
In the above described magnetic bearing apparatus, however, when a tool such as a grindstone or a cutter mounted on an end of the rotor is replaced on the way, the quantity of state such as the position of the center of gravity of the rotor is changed, so that position control becomes unstable, for example.
Therefore, there is provided a control device so adapted as to input the weight of a tool by inputting means when the tool is replaced to operate, for example, the position of the center of gravity (see Japanese Patent Laid-Open Gazette No. 97714/1990).
In the above described control device, however, the weight of the tool must be previously grasped, so that it takes a lot of time and labor to replace the tool.